Create a plane and apply ocean modifier to it
You need to add the ocean modifier before you can 'Apply' it. Try :

import bpy

model_height = 2

bpy.ops.mesh.primitive_plane_add(radius=10, view_align=False, location=(0, 0, 0))
bpy.ops.transform.translate(value=(0,0,-1.0 * model_height / 2.0 + 0.2 * model_height))

for obj in bpy.data.objects:
    obj.select = False

plane = bpy.data.objects['Plane']
plane.select = True
bpy.context.scene.objects.active = plane 

#Add the modifier
bpy.ops.object.modifier_add(type='OCEAN')

#Apply the modifier
bpy.ops.object.modifier_apply(modifier='Ocean')


Obviously, if you want to adjust the Ocean modifier you'll need to do this before 'apply'ing it (or don't apply it and leave it on the modifier stack).